Lyme disease (LD) is the most common tick-borne illness today in Europe and North America (Anderson et al. 1983, Burgdorfer et al. 1982, Johnson et al. 2014, Dryden et al. 2010). The Center for Disease Control (CDC) states that there may be more than 300,000 cases and as high as one million cases per year in United States alone (Stricker et al. 2014). This publication further states that Lyme disease is approaching an epidemic proportion and the monotherapy of using antibiotics may not be sufficient. Chronic/persistent Lyme disease involves latent forms such as rounded bodies and biofilm formation (Stricher et al. 2011).
Bacteria from genus Borrelia are pathogenic mico-aerophilic and slow-growing pathogens known for their persistency (Embers et al. 2012, Sapi et al. 2011, Hodzic et al. 2014, Stanek et al. 2012, Hodzic et al. 2008, Stricker et al. 2013, Barthold et al. 2010). Out of 36 currently recognized species, 17 of them are either acknowledged or suspected to cause LD. They are known as Borrelia burgdorferi sensu lato and include species such as Borrelia burgdorferi sensu stricto (predominantly causing this illness in North America) as well as Borrelia afzelii and Borrelia garinii (predominantly causing this illness in Europe) (Lovrich et al. 1994). The vegetative form of Borrelia sp. are spirochetes that are motile and can survive viscous conditions in human and animal blood, and as well they are capable of entering their cells (Miller et al. 2014, Zhao et al. 2014, Wu et al. 2011, Coleman et al. 1995). When hostile conditions are introduced these bacteria can adopt different latent persistent forms such as knob-shaped/rounded bodies (cysts, granular forms, L-forms) and aggregates (biofilm-like structures) both in vitro and in vivo (Gruntar et al. 2001, Brorson et al. 1998, Krause et al. 2006). Especially the ability of Borrelia burgdorferi sensu lato to convert and re-convert to cystic form was observed, which may be one of the reasons why this infection can sometimes become persistent and/or re-surface after being silent for a long time (Krause et al. 2006, Brorson et al. 1997, Donta 2002). Moreover, genomic analysis revealed that Borrelia burgdorferi sensu lato has a gene for efflux mechanism. This might be responsible for developing antibiotic resistance, although more in depth studies are warranted to prove such hypothesis (Fraser et al. 1997).
Based on diagnostic test results, the most recent estimates indicate that the number of LD cases in just the United States reaches 300,000 each year; however, there might be unreported cases that are not reflected in the statistics (Johnson et al. 2014, Krause et al. 2006, Stricker et al. 2014 Meek et al. 1996). This has made this disease the most common vector-born disease in the Northern Hemisphere (Johnson et al. 2014, Dryden et al. 2010, Krause et al. 2006). Currently, the frontline treatment for LD is based on antibiotics. Lactams and tetracyclines are the most frequently applied and include doxycycline, amoxicillin, and cefuroxime axetil. Macrolides, such as azithromycin, clarithromycin, and erythromycin, are the second class and appear to be less effective than lactams (Shapiro 2014).
Currently one of the most often prescribed antibiotics against Borrelia burgdorferi sensu lato infection is doxycycline (Shapiro 2014, Cameron et al. 2014, Nadelman et al. 2001, Wormser et al. 2006, Mygland et al. 2010). Its use began in the late sixties last century against infections other than LD (Li et al. 2013, Nelson et al. 2011). Since then, the application of doxycycline extended and has been on the World Health Organization's List of Essential Medicines as one of the most important medications needed in a basic health system (Nelson et al. 2011, World Health Organization 2015). Doxycycline is a broad-spectrum antibiotic belonging to the class of tetracyclines. Like other agents in this class, it is an anti-bacterial and anti-parasite agent targeting protein production in general (Nelson et al. 2011, World Health Organization 2015), The American Society of Health-System Pharmacists 2015). Its side effects are similar to those of other members of this antibiotic class, including the development of an erythematous rash after exposure to sun. Moreover, doxycycline is classified as a class D drug and is thus restricted for administration to pregnant women and children under the age of eight (Mylonas 2011). Although, doxycycline is one of the most frequently prescribed antibiotics, especially for the early stages of LD, its continued treatment is not recommended since the long-term effectiveness has not been proven (Stanek et al. 2012, Delong et al. 2012). In vitro studies revealed that doxycycline is effective against active (vegetative) form of Borrelia sp., with moderate action against their biofilms, and ineffective against the latent knob-shaped/rounded forms (Sapi et al. 2011, Baradaran-Dilmaghani et al. 1996). Observed persistency and/or relapse of LD symptoms in the absence of ongoing antibiotic treatment were observed as well (Krause et al. 2006, Donta 2002, Klempner et al. 2001). This would suggest that doxycycline may either facilitate generation of latent forms or is inefficient in their elimination (Hodzic et al. 2008, Gruntar et al. 2001, Oksi et al. 1999, Straubinger et al. 1997, Straubinger 2000).
Antimicrobials derived from natural sources such as plants, herbs, and fruits, and essential oils, have shown activity against a plethora of bacteria and fungi, but are poorly explored against Borrelia sp. (Morrison et al. 2014, Takeuchi et al. 2014, Zhang et al. 2013). Despite a rather small pool of available data on this subject, Bronson and Bronson study exhibited grapefruit seed extract as a powerful in vitro agent against spirochetes and their cystic forms of Borrelia burgdorferi sensu lato (Brorson et al. 2007). Dipsacus sylvestris extract against Borrelia burgdorferi sensu stricto revealed growth inhibiting activity as well (Liebold et al. 2011), whereas Sapi et al., reported significant efficacy of leaf extracts from Stevia Rebaudiana on all forms of Borrelia burgdorferi (Theophilus et al. 2015).
Natural components tested against Borrelia sp. were limited to individual compounds or extracts isolated from a specific plant species. There is a need for new and improved treatments against Borrelia sp.